U.S. patent number 5,548,449 [Application Number 08/255,930] was granted by the patent office on 1996-08-20 for objective-lens actuator.
This patent grant is currently assigned to NEC Corporation. Invention is credited to Yuki Fujimura, Tsutomu Matsui.
United States Patent |
5,548,449 |
Matsui , et al. |
August 20, 1996 |
Objective-lens actuator
Abstract
It is the object of the invention to provide the actuator for
the objective lens, suitable for the optical disc apparatus with
high speed access property and high drive sensitivity especially in
the track drive direction, and to solve problems concerning with
manufacturing processes, by using metallic leaf springs with high
reliability for supporting the lens holder to make its movements to
be translation motions in both focus and tracking drive directions.
The lens holder 22, a movable object, is supported by four leaf
springs 21a to 21d, thereby its movements is pure translation
motions in both focus and tracking drive directions. The drive
sensitivity in the track drive direction is largely increased by
designing and improving configurations of the magnetic circuit and
the focus and tracking drive coils. For tracking drive motion, a
pair of tracking drive coils are fixed to two opposite sides of the
focus drive coils, and the magnetic circuit contains two plate
magnets 15 and two bar magnets 16, such that the arrangement of
magnets is line symmetrically with respect to the axis of the
objective lens. The lens holder 22 is supported by the actuator
base 11 is provided with an air gap 30, the width of which is
adjustable by the aid of needle screws 19 and 20 to adjust the
inclination of the lens holder 22 in the tracking and tangential
directions respectively.
Inventors: |
Matsui; Tsutomu (Tokyo,
JP), Fujimura; Yuki (Tokyo, JP) |
Assignee: |
NEC Corporation (Tokyo,
JP)
|
Family
ID: |
15153130 |
Appl.
No.: |
08/255,930 |
Filed: |
June 7, 1994 |
Foreign Application Priority Data
|
|
|
|
|
Jun 7, 1993 [JP] |
|
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5-135497 |
|
Current U.S.
Class: |
359/814; 359/824;
369/44.15; G9B/7.082; G9B/7.083; G9B/7.084; G9B/7.085 |
Current CPC
Class: |
G11B
7/093 (20130101); G11B 7/0932 (20130101); G11B
7/0933 (20130101); G11B 7/0935 (20130101) |
Current International
Class: |
G11B
7/09 (20060101); G02B 007/02 () |
Field of
Search: |
;359/813,814,823,824
;369/44.15,44.16 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Ben; Loha
Attorney, Agent or Firm: Burns, Doane, Swecker & Mathis,
L.L.P.
Claims
What is claimed is:
1. An actuator for an objective lens, comprising:
a rectangular-shaped lens holder holding an objective lens at its
center,
a focus drive coil wound around outer periphery of said lens
holder, and generating magnetic flux in a direction of focus
drive,
pair of tracking drive coils fixed to opposite sides of said focus
drive coil near apexes, such that said tracking coils are line
symmetrical with respect to axis of said objective lens and
generate magnetic flux in a tangential direction perpendicular to
both said focus drive direction and a tracking drive direction,
four parallel leaf springs one end of each being fixed to said lens
holder and another end of each being fixed to a stand,
magnetic circuit assembly including permanent magnets and low
magnetic reluctance members, and generating a static magnetic field
in circumference of said lens holder and coils,
an actuator base supporting said magnetic circuit assembly and said
stand, such that said four leaf springs can adjust inclination of
said lens holder in both tracking drive and tangential
directions,
wherein, said static magnetic field is line asymmetrical with
respect to said lens axis, and in regions close to said opposite
sides of said focus coil, is in said tangential direction and its
magnitude is nearly constant along said opposite sides, however,
its direction is inverted at bisectors of said tracking drive
coils, and said low magnetic reluctance members and permanent
magnets hold said opposite sides of said focus drive coil between
them.
2. An actuator for an objective lens according to claim 1,
wherein:
said tracking drive coils are rectangular shaped.
3. An actuator for an objective lens according to claim 1, wherein
said permanent magnets include two linear arrays of a plate magnet
and a bar magnet which are magnetized in axial directions with
opposite polarities, and said linear arrays are positioned close to
and in parallel with said opposite sides of said focus drive coil
respectively, such that said two arrays are line symmetrical with
respect to said axis of said objective lens.
4. An actuator for an objective lens according to claim 1,
wherein:
said actuator base has rectangular-parallelepiped-formed portion
about its rear end, and is provided with a thin and
rectangular-shaped air gap running in parallel with its base plane,
and said stand is fixed to an upper portion of a rectangular
portion of said rectangular-parallelepiped-formed portion, and a
width of said air gap is adjustable by the aid of two needle screws
directed in tracking drive and tangential directions
respectively.
5. An actuator for an objective lens, comprising:
a lens holder for holding said objective lens;
focus and tracking drive coils mounted on said lens holder, wherein
the tracking drive coil comprises first and second flat and
rectangular coils provided on the focus coil to be positioned
opposite to one another relative to a central axis of the lens
holder;
a magnet assembly provided on a base, said magnet assembly
generating magnetic fields in opposite directions which are applied
to two opposite sides of the first and second flat and rectangular
coils, and said magnetic fields interacting with a magnetic flux
generated by an energized coil of said focus and tracking drive
coils, thereby generating at least one of focus and tracking forces
acting on said lens holder; and
a spring assembly for resiliently supporting said lens holder on
said base;
wherein said spring assembly, comprises: a pair of first parallel
springs each having a predetermined length, and first and second
ends; and a pair of second parallel springs each having said
predetermined length, and first and second ends; said first ends of
said first parallel springs being fixed to first and second points
on a top plane of said lens holder, said first ends of said second
parallel springs being fixed to first and second points on a bottom
plane of said lens holder, said first and second points on said top
and bottom planes being positioned opposite relative to said
central axis of said lens holder, said second ends of said first
and second parallel springs being fixed to first to fourth points
of said base.
6. An actuator for an objective lens, comprising:
a lens holder for holding said objective lens;
focus and tracking drive coils mounted on said lens holder;
a magnet assembly provided on a base, said magnet assembly
generating magnetic fields in opposite directions, and said
magnetic fields interacting with a magnetic flux generated by an
energized coil of said focus and tracking drive coils, thereby
generating at least one of focus and tracking forces acting on said
lens holder; and
a spring assembly for resiliently supporting said lens holder on
said base;
wherein said focus drive coil is on a circumferential outer surface
of said lens holder;
said tracking drive coil comprises first and second flat and
rectangular coils provided on said focus coil to be positioned
opposite relative to a central axis of said lens holder; and
said magnet assembly consists of first and second magnets
positioned opposite relative to said central axis, each of said
first and second magnet consisting of a wide magnet and a narrow
magnet, said narrow magnet facing a half portion of a corresponding
one of said first and second flat and rectangular coils, while said
wide magnet facing a remaining half portion of said corresponding
one and a portion of said focus coil.
Description
FIELD OF THE INVENTION
The invention relates to an objective-lens actuator for an optical
head which is controlled to move in focus and tracking directions
with high information bit rate recording.
BACKGROUND OF THE INVENTION
An optical disc apparatus for the next generation is required to
operate with a large capacity of information and the high
information bit rate recording. For this purpose, an objective lens
for high information bit rate recording is used for an optical
head, and is required to be controlled with a high speed and the
high precision. Therefore, an objective lens actuator to be moved
in focus and track directions must be small in size and light in
weight, and must generate a driving force to overcome the disc
acceleration.
As an example of conventional apparatus, we can quote the objective
lens actuator which appeared on Japanese Patent Kokai No. 3-156730.
The outline of the aforementioned apparatus is as follows. A focus
drive coil is a rectangular-shaped one and wound around on outer
periphery of a lens holder. A pair of parallel tracking drive coils
are fixed to opposite sides of a focus drive coil, such that coils
surface of the formers and the latter are perpendicular to each
other.
The lens holder and aforementioned coils are placed in DC magnetic
field caused by a pair of parallel plate magnets which are fixed to
the actuator base and magnetized in opposite directions.
In operation, the current in the focus drive coil is proportional
to the focus servo error signal corresponding to the focus drive
direction, and the electro-magnetic force which is generated by
this coils is proportional to the product of the coil current and
the external magnetic flux density, i.e. the one caused by
aforementioned plate magnets, and the direction of the
electro-magnetic force is perpendicular to the coil surface.
Therefore, the focus drive coil moves up and down corresponding to
the polarity of the coil current, until the electro-magnetic force
balances with restoring forces of four wires which support the lens
holder. In the case of the tracking drive coils, all situations are
quite similar to those of the focus drive one, however, the coils
moves to the right and the left according to the servo error signal
corresponding to the tracking drive direction. Thus, focus and
tracking adjustments are carried out in accordance with servo error
signals supplied from an optical unit, which contains various
optical hardwares and produces focus and tracking error signals,
and information signals.
On the aforementioned objective lens actuator, however, following
disadvantages have been pointed out. The structure for
four-wire-suspension which was designed to move objectives lens in
two orthogonal axes causes rotational motions in focus and tracking
directions. Moreover, for objective-lens actuator, it is difficult
to increase the drive sensitivity of tracking drive coils, because
dimensions of these coils and external DC magnetic fields acting to
these coils cannot be so large, by the reasons of spacial
limitations and structures of magnetic circuits respectively.
Furthermore, a mechanism for adjusting initial position and
attitude of the lens holder in both tracking drive and tangential
(perpendicular to tracking) direction becomes complicated, when
larger range of adjustment is desired.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide an
actuator for an objective lens in which the drive sensitivity of
the tracking drive is remarkably improved.
It is a further object of the invention to provide an actuator for
an objective lens in which forces act to the lens holder cause only
its translation motions in both focus and tracking drive
directions.
It is a still further object of the invention to provide an
objective lens actuator in which the initial position and the
initial attitude of the lens holder can be easily adjusted by
simple mechanism.
According to the invention, an actuator for an objective lens,
comprises:
a rectangular-shaped lens holder holding an objective lens at its
center,
a focus drive coil wound around outer periphery of the lens holder,
and generating magnetic flux in a direction of focus drive,
pair of tracking drive coils fixed to opposite sides of the focus
drive coil near around apexes, such that the tracking coils are
line symmetrical with respect to axis of the objective lens and
generate magnetic flux in a direction perpendicular to both the
focus and tracking drive directions (tangential direction,
hereinafter),
four parallel leaf springs one ends of which are fixed to the lens
holder and another ends of which to a stand,
magnetic circuit assembly consisted of permanent magnets and low
magnetic reluctance members, and generating static magnetic field
in circumference of the lens holder and coils,
an actuator base supporting the magnetic circuit assembly and the
stand, such that the four leaf springs can adjust inclination of
the lens holder in both tracking drive and tangential
directions,
wherein, the static magnetic field is line antisymmetrical with
respect to the lens axis, and in regions close to the opposite
sides of the focus coil, points to tangential direction and its
magnitude is nearly constant along the opposite sides, however, its
direction is inverted at bisectors of the tracking drive coils, and
the low magnetic reluctance members and permanent magnets hold the
opposite sides of the focus drive coil between them.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be explained in more detail in conjunction with
appended drawings, wherein:
FIG. 1 is a diagram showing a whole construction and a mechanism
for adjusting inclination of a lens holder in a conventional
actuator.
FIG. 2A and 2C show magnetic flux distributions around focus and
tracking drive coils in a conventional apparatus, respectively,
FIG. 2B and 2D show relations between magnetic fields, coil
currents and electro-magnetic forces in cases of focus and tracking
drive coils in a conventional apparatus respectively,
FIG. 3 is a disassembled and perspective diagram showing an
actuator for an objective lens in a preferred embodiment according
to the invention,
FIGS. 4A to 4C are plan views of the one of four leaf springs under
various conditions,
FIGS. 5A to 5C are a plan, an elevation and a side views of an
embodiment according to the invention shown in FIG. 3,
FIG. 6 shows relations between magnetic fields, electric current
and the electro-magnetic force acting to the tracking drive coil,
and
FIG. 7 shows the mechanism for adjusting the inclination of the
lens holder in the tangential direction.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Before explaining an actuator for an objective lens in a preferred
embodiment according to the invention, the aforementioned
conventional actuator for the objective lens will be again
explained referring to FIG. 1. In the upper portion of FIG. 1,
three directions, that is to say, the focus drive, the tracking
drive, and the tangential directions, are shown by arrows. The
focus drive direction is perpendicular to the optical disc (not
shown), and coincides with the objective lens axis, the tracking
drive direction is the radial one of the disc (not shown), and the
tangential direction is the one of the tangent of the disc. In
operation, the lens holder is controlled by the actuator in both
the focus and the tracking drive directions; in manufacturing
process, however, the initial position and the initial attitude of
the lens holder are adjusted manually in the tracking drive and
tangential directions.
As shown in FIG. 1, the holder 41 is approximately
rectangular-shaped and hold the objective lens at its center. The
focus drive coil 42 is wound around the outer periphery of the lens
holder 41, and a pair of rectangular-shaped tracking drive coils 43
are fixed to both ends of lens holder, such that coils surfaces of
those coils are parallel with each other and perpendicular to that
of the focus drive coil. 33a and 33b are plate magnets, and
magnetized in the direction of the depth, in other words,
perpendicular to plate surfaces. These plate magnet 33a and 33b are
arranged in parallel with each other but with opposite directions
of magnetization, positioned in parallel with and close by opposite
sides of the focus drive coil 42 respectively, and fixed to the
actuator base 34. The lens holder 41 are also supported by the
actuator base 34 through four wires 44 and four-wires-supporting
member 46. The left end of each one of four wires is fixed to the
lens holder 41 at four-wires-fixing positions 45, and the right end
to four-wires-supporting member 46, which are supported by actuator
base 34 using three screws 4a and a coil spring 47. The initial
position and the initial attitude of the lens holder 41 can be
adjusted in the tracking drive or tangential direction by three
screws 48. 49a and 49b in FIG. 1 are magnetic flux guiding yokes
(flux guide, hereinafter), and made of magnetic material.
FIG. 2B is a plan view of the focus drive coil 42, where H is the
external magnetic field acts to this coil, and I is the electric
current in the coil conductor. F is the electro-magnetic force acts
to the coil, the direction of which is perpendicular to the coil
surface and points to the reader according to the Fleming's left
hand law. F is proportional to the product of I and H.
FIG. 2D is an elevation view of the tracking coil 43 and the
magnetic flux meanings of H, I and F are quite similar to those in
FIG. 2B.
FIG. 2A is a cross-sectional view corresponding to FIG. 2B in the
A--A cross section, where .phi. means the magnetic flux which flows
from the flux guide 49a to the plate magnet 33a. It can be easily
understood that the magnetic flux density is largely increased near
the flux guide 49a. Because the actuator base 34 is a part of the
magnetic circuitry, the electro-magnetic force act acting to the
electric current in the focus drive coil 42 is also increased.
FIG. 2C is a cross-sectional view corresponding to FIG. 2B in the
B--B cross-section. The region shown in this drawing is near by the
tracking drive coil 43 and the plate magnet 33a is remoted from the
flux guide 49a, and thereby flux density is thin in this region.
Accordingly, the electro-magnetic force act to the tracking drive
coil 43 is small. Moreover, the dimension of the tracking drive
coil cannot be large by spacial limitations, therefore, the drive
sensitivity in the tracking drive direction is unsatisfactory.
Moreover, a larger space is necessary for the four-wire-system for
adjusting the initial position and the initial attitude of the lens
holder in both tracking drive and tangential directions, and this
system is complicated and unsuitable for the manufacturing
process.
Next, an actuator for an objective lens in the preferred embodiment
according to the invention will be explained referring to FIGS. 3
to 7.
In an upper portion of FIG. 3, the focus and the tracking drive
directions are shown by two arrows respectively. A lens holder 22
is approximately rectangular-shaped and holds the objective lens 26
at its center. A focus drive coil 23 is wound around the outer
periphery of the lens holder 22. A pair of tracking drive coils 24
and 25 are rectangular-shaped and fixed to opposite sides of the
focus drive coil 23 near around apexes, such that the arrangement
of these coils is line symmetrical with respect to the axis of the
objective lens 26. An actuator base 11, which supports various
elements thereon, is made of magnetic material, and is a part of
magnetic circuit. Rectangular-parallelepiped portions at both ends
are also involved in the actuator base 11. A collimator beam 31
passes a circular aperture 12 set on the actuator base 11 as shown
in FIG. 5B. 15a and 15b are plate magnets magnetized in the depth
direction with opposite polarities. 16a and 16b are bar magnets
magnetized in the depth direction with opposite polarities. The
polarities of magnetization of the plate magnet 15a and the bar
magnet 16a are opposite to each other.
These magnets compose a linear array as shown in FIG. 3. The
similar relations hold also between the plate magnet 15b and the
bar magnet 16b. These magnet arrays are arranged to be line
symmetrical with respect to the axis of the objective lens 26. 14a
and 4b are flux guides (magnetic flux guide members) with low
magnetic reluctances and slightly shorter lengths than those of
said opposite sides of the focus drive coil 23. Lower ends of flux
guide 14a and 14b are fixed to the actuator base 11. 13a and 13b
are magnetic gap and conductors of focus drive coils 23 and
tracking drive coils 24 and 25 are positioned in these places when
assembled.
The lens holder 22 are supported by four leaf springs 21a to 21d.
The right and the left ends of these leaf springs are fixed to the
lens holder 22 and a stand 17 respectively, by mean of adhesive or
soldering. The stand 17 is fixed to the actuator base 11 by fixing
screws 32.
Next, forces act to coils will be explained. FIG. 6 is a cross
sectional view which shows the relation between the magnetic field
distribution in the magnetic gap 13a and the electro-magnetic force
acting to the tracking drive coil 24. The focus drive coil 23 is
not shown for simplicity. The chain line is a bisector line of the
space between the lower end of the plate magnet 15a and the upper
end of the bar magnet 16a, and its extension line intersects the
bisector of the tracking drive coil 24. The magnetic field H in the
magnetic gap 13a is shown as vectors in FIG. 6, and is
perpendicular to surface planes and has nearly equal magnitude,
because magnitude of magnetization is uniform all over surfaces of
magnets. The polarity of magnetization, however, is inverted in the
bar magnet 16a, the direction of magnetic field in the vicinity of
this magnet is also inverted as shown in FIG. 6. On the other hand,
the coil current in this region is -I, therefore the
electro-magnetic forces acting to upper and lower currents in FIG.
6 point to the same direction, namely the tracking drive
directions, according to the Fleming's left hand law. The magnetic
field near by the tracking drive coil is largely increased as
compared with that in the case of FIG. 2D because of the flux guide
14a which covers whole area of the tracking drive coil 24. The same
result can be obtained in the tracking drive coil 25, therefore the
satisfactory drive sensitivity can be achieved in the tracking
drive.
In the case of focus drive coil, all situations are quite similar
to the case of FIG. 2B, except the existence of the bar magnets 16a
and 16b, which decrease the electro-magnetic force in the focus
drive direction to a small extent, and therefore the detailed
explanations will be omitted.
FIG. 4A shows one of leaf springs 21a to 21d in the case of no
external force. The left end and the right one of each spring have
an elliptical hole 27 and a circular one 28 respectively. These
leaf springs are made of metal and have high reliability as
compared with conventional ones made of plastics. The position and
the attitude of the lens holder 22, which is supported by four leaf
springs 21a to 21d, relative to the stand 17 is established by
jigs, and elliptical hole 28 is used as play in this work. Near the
both ends of the leaf spring, there are rectangular shaped frames,
by which stiffness of the spring is increased at these portions.
The spring is smoothly bent not only in the normal direction (the
focus drive direction) but also in the lateral direction (the
tracking drive direction). FIG. 4B shows the situation of a common
spring without the frames under the same condition as that of FIG.
4B.
In a leaf spring according to the invention, compliance is larger,
and linear relation between stress and strain holds in wide range
of forces especially of lateral force.
The assembled actuator is shown in FIGS. 5A to 5C, which are plan,
elevation and side views respectively. 31 is a parallel beam by
light collimated by a collimator lens (not shown), which passes
through the circular aperture 12 and is focused by the objective
lens 26.
The actuator base 11 has a rectangular-parallelepiped portion near
its rear end, which is provided with a rectangular-shaped thin air
gap 30, and the upper and the lower portions are denoted by 11a and
11b respectively. The stand 17 is fixed to the upper portion 11a by
fixing screws 32. 19 and 20 are needle screws with cone-shaped
leading ends. The leading-ends of the needle screws 19 and 20
thrust into the air gap 30, and expand its gap-distances, and
thereby adjust inclinations of the stand 17 in both the tracking
drive and the tangential directions respectively and independently.
For more detailed explanations, FIG. 7 shows the cross sectional
view of the rear end of the actuator base 11. A female screw is
provided in the rear end of the actuator 11, which guides the
needle screw 20 into the inner region. This screw 20 is driven or
loosened by a driver through a hole 18. The configuration
surrounding the needle screw 19 is quite similar to that of the
needle screw 20. The inclinations of the stand 17 are transmitted
to the lens holder 22 through four leaf springs 21a to 21d.
In the above explanations, the tracking drive coil 24 and 25 are
rectangular shaped. But similar effects can be achieved by any coil
with line symmetrical shape, such as circular or elliptical
one.
As described above, in the actuator for the objective lens
according to the invention, movements of the lens holder are pure
translation motions in both the focus drive and the tracking drive
directions, because of metallic leaf springs with high reliability,
and the stability of the objective lens is largely improved. The
drive sensitivity in the tracking drive direction is increased by
improving the configuration of driving coils and magnetic circuits.
Initial adjustments of inclinations of the lens holder are
simplified, and various problems concerning manufacturing processes
can be solved.
Although the invention has been described with respect to specific
embodiments for complete and clear disclosure, the appended claims
are not to be thus limited but are to be construed as embodying all
modification and alternative constructions that may occur to one
skilled in the art.
* * * * *